1. Technical Field of the Invention
The present invention relates to a power unit case for a power unit including an internal combustion engine and a transmission. In the power unit case, a crankcase and a transmission case are formed integrally together as a power unit case. The present invention also relates to a power unit incorporating an engine and a transmission within the described power unit case.
2. Background Art
With regard to internal combustion engines, used as a power source for vehicles, including a unitary case in which the rotation of a crankshaft in a crankcase is transmitted to the rotation of a main shaft in a transmission case via the meshing of primary gears, a combination of transmission gears for effectively transmitting power is selected and set from among a group of transmission gears on the main shaft and a group of transmission gears on the counter shaft, and the rotation of the main shaft is transmitted to the rotation of the counter shaft at a gear ratio of the set combination of transmission gears is generally known.
In addition, it is known to use a hollow lubricating-oil passage formed around the axial center of each of the main shaft and counter shaft where lubricating oil is ejected radially from oil holes formed at several locations of this lubricating-oil passage, thereby lubricating the group of transmission gears in the transmission chamber (see published patent document JP-2004-290662, for example).
In the case of the internal combustion engine as disclosed in published patent document JP-2004-290662, the unitary case that integrally forms the crankcase and the transmission case is formed so as to be divided into an upper case and a lower case, with the crankshaft, the main shaft, and the counter shaft arranged in the parting surface of the unitary case. Ribs that obliquely cross each other are formed in the inner surface above the transmission chamber of the upper case, and the intersection of the ribs is positioned directly above a predetermined meshing portion of transmission gears, so that lubricating oil thrown upward by the rotation of the group of transmission gears adheres onto the inner surface above the transmission chamber of the upper case, and the lubricating oil that has flown along the inner surface is dripped to the predetermined meshing portion of transmission gears from the intersection of the ribs, thereby effecting lubrication.
The unitary case of the internal combustion engine disclosed in published patent document JP-2004-290662 has a structure with a long front-to-rear width in which the crankshaft, the main shaft, and the counter shaft are arranged side by side in the front-to-rear direction in this order in the parting surface between the upper and lower unitary case halves. Although the arrangement of the main shaft and the counter shaft side by side in the front-to-rear direction allows lubricating oil to be dripped to the meshing portion of transmission gears from above, the front-to-rear width of the unitary case becomes large, resulting in an increase in the size of the internal combustion engine.